1. Field of the Invention
The present invention relates to an AC motor starting system and method and, more particularly, a multiple AC motor starting system and method.
2. Related Prior Art
Multiple AC motor starting systems are known. One prior art system is disclosed in Soviet Union Pat. No. 1822318 A1.
In the motor starting method and system disclosed in the SU patent, the motor stator windings of two motors are connected in series and the voltage of the power network is applied to them. The result vector of the power network is less than the sum of the rated voltage of each motor. After the motors reach a certain speed, the rated voltage is applied to each motor. This is achieved by creating a neutral point at the neutral side of the motors. The voltage result vector, which is less than the sum of the rated voltage of each motor, is the line to line voltage of the power network.
To decrease the starting current, current limiting resistors or reactors are inserted in series with the motors before applying the voltage of the power network. The resistors are shunted out of the circuit after applying this voltage when the motors have reached a certain speed.
The system includes a first motor having phase leads of the stator windings connected to the power network through a switching means, such as a circuit breaker, contactor or other switching device, and a second motor with phase leads of the stator windings connected to the power network by another switching means. The phase leads of the two motors are transposed with respect to one another. Another switching means is connected to the neutral ends of the motors at the point of their series connection to create a neutral point for both motors.
This prior art starting method and system has a number of problems.
First, since the phase to phase voltage is applied to the two motors connected in series, the starting current can only be reduced to 86.5% of the starting current which would occur when the full line voltage is applied to the motor. Reducing the starting current to less than 86.5% requires the use of additional current limiting resistors or reactors and switching means to shunt them out of the circuit when the motors reach a certain speed.
Second, the prior art method and starting system is only useful in applications with substantially identical motors having similar starting impedances.
Third, the number of motors, which can be started simultaneously with reduced starting current, does not exceed two. To reduce the starting current in the system with more than two motors, additional current limiting resistors or reactors and switching means must be used.
Furthermore, this method and system can be used only to start up AC motors designed to work continuously with star type connections of the stator windings. The method and system cannot be used for delta type stator winding connections of AC motors, which are most commonly used in industrial applications.
The primary objective of the present invention is to provide a method and system for starting multiple AC motors simultaneously while maintaining a lower start-up current than was possible with prior art designs.
Another object of the invention is to provide a start-up circuit for AC motors having delta type stator winding connections where the start-up current is substantially reduced.
A further object of the invention is to provide a multiple AC motor start-up circuit capable of simultaneously starting multiple motors of varying sizes and using a reduced start-up current.
According to the invention there is provided a method of start-up of AC motors in a multi-motor system and a system to perform this method comprising the connecting of the motor stator windings in series or, where more than two motors are being started, in parallel/series, applying the voltage of the power network to the motors, the result vector of which is less than the sum of the rated voltage of each motor, and, after a preset time delay or when the motors reach a certain speed, applying the rated voltage to each motor by disconnecting the stator windings from the series connection and creating a delta type connection of the stator windings.
The system which performs this method comprises one motor having phase leads of the stator windings connected to the power network by switching means such as a circuit breaker, contactor or other switching device, and another motor having phase leads of stator windings connected to the power network by another switching means where the phase leads are circularly transposed. A third switching means is installed in series between the stator windings of the motors. Two further switching means are connected to both sides of each motor in order to create the delta type connection of the motor stator windings.
A further feature of the invention is that by providing an additional switching means connected to the series connection between the stator windings on one side and connected together on the other side to provide a neutral point, the motors can be started up individually with a Wye connection and, when they reach a certain speed, are converted to the delta running connection desired.
An additional resistor or reactor may be installed in series with the switching means installed between the motor stator windings and the point of their series connection to further reduce the starting current.
Where more than two motors are to be started at one time, the additional motors can be paired in parallel sets which then have their stator windings connected in series with the stator windings of another parallel pair of motors.